Aperiodic tracking reference signal

ABSTRACT

Certain aspects of the present disclosure provide techniques for determining when to transmit and transmitting an aperiodic tracking reference signal (TRS). Certain aspects provide a method for wireless communication. The method generally includes determining a channel condition change of a downlink channel between a first wireless device and a second wireless device. The method further includes determining whether the channel condition change satisfies a trigger condition. The method further includes triggering transmission of an aperiodic tracking reference signal when the channel condition change satisfies the trigger condition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 16/162,078, filed Oct. 16, 2018, which claims the benefit of U.S. Provisional Patent No. 62/574,185, filed Oct. 18, 2017. The contents of each of these applications are hereby incorporated by reference in their entirety.

INTRODUCTION

Aspects of the present disclosure relate to wireless communications, and more particularly, to techniques for determining when to transmit and transmitting an aperiodic tracking reference signal (TRS).

Wireless communication systems are widely deployed to provide various telecommunication services such as telephony, video, data, messaging, and broadcasts. Typical wireless communication systems may employ multiple-access technologies capable of supporting communication with multiple users by sharing available system resources (e.g., bandwidth, transmit power). Examples of such multiple-access technologies include Long Term Evolution (LTE) systems, code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, orthogonal frequency division multiple access (OFDMA) systems, single-carrier frequency division multiple access (SC-FDMA) systems, and time division synchronous code division multiple access (TD-SCDMA) systems.

In some examples, a wireless multiple-access communication system may include a number of base stations, each simultaneously supporting communication for multiple communication devices, otherwise known as user equipment (UEs). In LTE or LTE-A network, a set of one or more base stations may define an eNodeB (eNB). In other examples (e.g., in a next generation or 5G network), a wireless multiple access communication system may include a number of distributed units (DUs) (e.g., edge units (EUs), edge nodes (ENs), radio heads (RHs), smart radio heads (SRHs), transmission reception points (TRPs), etc.) in communication with a number of central units (CUs) (e.g., central nodes (CNs), access node controllers (ANCs), etc.), where a set of one or more distributed units, in communication with a central unit, may define an access node (e.g., a new radio base station (NR BS), a new radio node-B (NR NB), a network node, 5G NB, gNB, etc.). A base station or DU may communicate with a set of UEs on downlink channels (e.g., for transmissions from a base station or to a UE) and uplink channels (e.g., for transmissions from a UE to a base station or distributed unit).

These multiple access technologies have been adopted in various telecommunication standards to provide a common protocol that enables different wireless devices to communicate on a municipal, national, regional, and even global level. An example of an emerging telecommunication standard is new radio (NR), for example, 5G radio access. NR is a set of enhancements to the LTE mobile standard promulgated by Third Generation Partnership Project (3GPP). It is designed to better support mobile broadband Internet access by improving spectral efficiency, lowering costs, improving services, making use of new spectrum, and better integrating with other open standards using OFDMA with a cyclic prefix (CP) on the downlink (DL) and on the uplink (UL) as well as support beamforming, multiple-input multiple-output (MIMO) antenna technology, and carrier aggregation.

However, as the demand for mobile broadband access continues to increase, there exists a need for further improvements in NR technology. Preferably, these improvements should be applicable to other multi-access technologies and the telecommunication standards that employ these technologies.

BRIEF SUMMARY

The systems, methods, and devices of the disclosure each have several aspects, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this disclosure as expressed by the claims which follow, some features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled “Detailed Description” one will understand how the features of this disclosure provide advantages that include improved communications between access points and stations in a wireless network.

Certain aspects provide a method for wireless communication. The method generally includes determining a channel condition change of a downlink channel between a first wireless device and a second wireless device. The method further includes determining whether the channel condition change satisfies a trigger condition. The method further includes triggering transmission of an aperiodic tracking reference signal when the channel condition change satisfies the trigger condition.

Certain aspects provide a first wireless device including a memory and a processor coupled to the memory. The processor is configured to determine a channel condition change of a downlink channel between the first wireless device and a second wireless device. The processor is further configured to determine whether the channel condition change satisfies a trigger condition. The processor is further configured to trigger transmission of an aperiodic tracking reference signal when the channel condition change satisfies the trigger condition.

Certain aspects provide a first wireless device. The first wireless device includes means for determining a channel condition change of a downlink channel between the first wireless device and a second wireless device. The first wireless device further includes means for determining whether the channel condition change satisfies a trigger condition. The first wireless device further includes means for triggering transmission of an aperiodic tracking reference signal when the channel condition change satisfies the trigger condition.

Certain aspects provide a non-transitory computer readable medium that stores instructions that when executed by a first wireless device cause the first wireless device to perform a method of wireless communication. The method generally includes determining a channel condition change of a downlink channel between a first wireless device and a second wireless device. The method further includes determining whether the channel condition change satisfies a trigger condition. The method further includes triggering transmission of an aperiodic tracking reference signal when the channel condition change satisfies the trigger condition.

Certain aspects provide a method for wireless communication. The method generally includes determining whether a trigger condition is satisfied. The method further includes triggering transmission of an aperiodic tracking reference signal from a base station to a user equipment when the trigger condition is satisfied, wherein the base station is further configured to transmit periodically a periodic tracking reference signal to the user equipment, wherein the aperiodic tracking reference signal and the periodic tracking reference signal have a same structure.

Certain aspects provide a wireless device including a memory and a processor coupled to the memory. The processor is configured to determine whether a trigger condition is satisfied. The processor is further configured to trigger transmission of an aperiodic tracking reference signal from a base station to a user equipment when the trigger condition is satisfied, wherein the base station is further configured to transmit periodically a periodic tracking reference signal to the user equipment, wherein the aperiodic tracking reference signal and the periodic tracking reference signal have a same structure.

Certain aspects provide a wireless device. The wireless device includes means for determining whether a trigger condition is satisfied. The wireless device further includes means for triggering transmission of an aperiodic tracking reference signal from a base station to a user equipment when the trigger condition is satisfied, wherein the base station is further configured to transmit periodically a periodic tracking reference signal to the user equipment, wherein the aperiodic tracking reference signal and the periodic tracking reference signal have a same structure.

Certain aspects provide a non-transitory computer readable medium that stores instructions that when executed by a wireless device cause the wireless device to perform a method of wireless communication. The method generally includes determining whether a trigger condition is satisfied. The method further includes triggering transmission of an aperiodic tracking reference signal from a base station to a user equipment when the trigger condition is satisfied, wherein the base station is further configured to transmit periodically a periodic tracking reference signal to the user equipment, wherein the aperiodic tracking reference signal and the periodic tracking reference signal have a same structure.

To the accomplishment of the foregoing and related ends, the one or more aspects comprise the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative features of the one or more aspects. These features are indicative, however, of but a few of the various ways in which the principles of various aspects may be employed, and this description is intended to include all such aspects and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the present disclosure can be understood in detail, a more particular description, briefly summarized above, may be had by reference to aspects, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only certain typical aspects of this disclosure and are therefore not to be considered limiting of its scope, for the description may admit to other equally effective aspects.

FIG. 1 is a block diagram conceptually illustrating an example telecommunications system, in accordance with certain aspects of the present disclosure.

FIG. 2 is a block diagram illustrating an example logical architecture of a distributed RAN, in accordance with certain aspects of the present disclosure.

FIG. 3 is a diagram illustrating an example physical architecture of a distributed RAN, in accordance with certain aspects of the present disclosure.

FIG. 4 is a block diagram conceptually illustrating a design of an example BS and user equipment (UE), in accordance with certain aspects of the present disclosure.

FIG. 5 is a diagram showing examples for implementing a communication protocol stack, in accordance with certain aspects of the present disclosure.

FIG. 6 illustrates an example of a DL-centric subframe, in accordance with certain aspects of the present disclosure.

FIG. 7 illustrates an example of an UL-centric subframe, in accordance with certain aspects of the present disclosure.

FIG. 8 illustrates an example timeline of channel conditions of a DL between a user equipment (UE) and a base station (BS), in accordance with certain aspects of the present disclosure.

FIG. 9 illustrates an example timeline of channel conditions of a DL between a UE and a BS, in accordance with certain aspects of the present disclosure.

FIG. 10 illustrates example operations that may be performed by a wireless device for using an aperiodic TRS in accordance with aspects of the present disclosure.

FIG. 11 illustrates a communications device that may include various components configured to perform operations for the techniques disclosed herein in accordance with aspects of the present disclosure.

To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one aspect may be beneficially utilized on other aspects without specific recitation.

DETAILED DESCRIPTION

Aspects of the present disclosure provide apparatus, methods, processing systems, and computer readable mediums for NR (new radio access technology or 5G technologies).

NR may support various wireless communication services, such as Enhanced mobile broadband (eMBB) targeting wide bandwidth (e.g. 80 MHz beyond), millimeter wave (mmW) targeting high carrier frequency (e.g. 27 GHz or beyond), massive MTC (mMTC) targeting non-backward compatible MTC techniques, and/or mission critical targeting ultra reliable low latency communications (URLLC). These services may include latency and reliability requirements. These services may also have different transmission time intervals (TTI) to meet respective quality of service (QoS) requirements. In addition, these services may co-exist in the same subframe. In LTE, the basic transmission time interval (TTI) or packet duration is 1 subframe. In NR, a subframe may still be 1 ms, but the basic TTI may be referred to as a slot. A subframe may contain a variable number of slots (e.g., 1, 2, 4, 8, 16, . . . slots) depending on the tone-spacing (e.g., 15, 30, 60, 120, 240 . . . kHz).

Aspects of the present disclosure relate to aperiodic tracking reference signals.

The following description provides examples, and is not limiting of the scope, applicability, or examples set forth in the claims. Changes may be made in the function and arrangement of elements discussed without departing from the scope of the disclosure. Various examples may omit, substitute, or add various procedures or components as appropriate. For instance, the methods described may be performed in an order different from that described, and various steps may be added, omitted, or combined. Also, features described with respect to some examples may be combined in some other examples. For example, an apparatus may be implemented or a method may be practiced using any number of the aspects set forth herein. In addition, the scope of the disclosure is intended to cover such an apparatus or method which is practiced using other structure, functionality, or structure and functionality in addition to or other than the various aspects of the disclosure set forth herein. It should be understood that any aspect of the disclosure disclosed herein may be embodied by one or more elements of a claim. The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any aspect described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects.

The techniques described herein may be used for various wireless communication networks such as LTE, CDMA, TDMA, FDMA, OFDMA, SC-FDMA and other networks. The terms “network” and “system” are often used interchangeably. A CDMA network may implement a radio technology such as Universal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includes Wideband CDMA (WCDMA) and other variants of CDMA. cdma2000 covers IS-2000, IS-95 and IS-856 standards. A TDMA network may implement a radio technology such as Global System for Mobile Communications (GSM). An OFDMA network may implement a radio technology such as NR (e.g. 5G RA), Evolved UTRA (E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Flash-OFDMA, etc. UTRA and E-UTRA are part of Universal Mobile Telecommunication System (UMTS). NR is an emerging wireless communications technology under development in conjunction with the 5G Technology Forum (5GTF). 3GPP Long Term Evolution (LTE) and LTE-Advanced (LTE-A) are releases of UMTS that use E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A and GSM are described in documents from an organization named “3rd Generation Partnership Project” (3GPP). cdma2000 and UMB are described in documents from an organization named “3rd Generation Partnership Project 2” (3GPP2). The techniques described herein may be used for the wireless networks and radio technologies mentioned above as well as other wireless networks and radio technologies. For clarity, while aspects may be described herein using terminology commonly associated with 3G and/or 4G wireless technologies, aspects of the present disclosure can be applied in other generation-based communication systems, such as 5G and later, including NR technologies.

Example Wireless Communications System

FIG. 1 illustrates an example wireless communication network 100 in which aspects of the present disclosure may be performed. For example, the wireless network may be a new radio (NR) or 5G network. UEs 120 and/or BSs 110 may be configured to perform the operations 1000 and methods described herein for using an aperiodic TRS. The UEs 120 and/or BSs 110 may further be configured to perform complementary operations to the operations 1000.

As illustrated in FIG. 1, the wireless network 100 may include a number of BSs 110 and other network entities. A BS may be a station that communicates with UEs. Each BS 110 may provide communication coverage for a particular geographic area. In 3GPP, the term “cell” can refer to a coverage area of a Node B and/or a Node B subsystem serving this coverage area, depending on the context in which the term is used. In NR systems, the term “cell” and gNB, Node B, 5G NB, AP, NR BS, NR BS, or TRP may be interchangeable. In some examples, a cell may not necessarily be stationary, and the geographic area of the cell may move according to the location of a mobile BS. In some examples, the base stations may be interconnected to one another and/or to one or more other BSs or network nodes (not shown) in the wireless communication network 100 through various types of backhaul interfaces such as a direct physical connection, a virtual network, or the like using any suitable transport network.

In general, any number of wireless networks may be deployed in a given geographic area. Each wireless network may support a particular radio access technology (RAT) and may operate on one or more frequencies. A RAT may also be referred to as a radio technology, an air interface, etc. A frequency may also be referred to as a carrier, a frequency channel, etc. Each frequency may support a single RAT in a given geographic area in order to avoid interference between wireless networks of different RATs. In some cases, NR or 5G RAT networks may be deployed.

A BS may provide communication coverage for a macro cell, a pico cell, a femto cell, and/or other types of cell. A macro cell may cover a relatively large geographic area (e.g., several kilometers in radius) and may allow unrestricted access by UEs with service subscription. A pico cell may cover a relatively small geographic area and may allow unrestricted access by UEs with service subscription. A femto cell may cover a relatively small geographic area (e.g., a home) and may allow restricted access by UEs having association with the femto cell (e.g., UEs in a Closed Subscriber Group (CSG), UEs for users in the home, etc.). A BS for a macro cell may be referred to as a macro BS. A BS for a pico cell may be referred to as a pico BS. A BS for a femto cell may be referred to as a femto BS or a home BS. In the example shown in FIG. 1, the BSs 110 a, 110 b and 110 c may be macro BSs for the macro cells 102 a, 102 b and 102 c, respectively. The BS 110 x may be a pico BS for a pico cell 102 x. The BSs 110 y and 110 z may be femto BS for the femto cells 102 y and 102 z, respectively. A BS may support one or multiple (e.g., three) cells.

The wireless communication network 100 may also include relay stations. A relay station is a station that receives a transmission of data and/or other information from an upstream station (e.g., a BS or a UE) and sends a transmission of the data and/or other information to a downstream station (e.g., a UE or a BS). A relay station may also be a UE that relays transmissions for other UEs. In the example shown in FIG. 1, a relay station 110 r may communicate with the BS 110 a and a UE 120 r in order to facilitate communication between the BS 110 a and the UE 120 r. A relay station may also be referred to as a relay BS, a relay, etc.

The wireless network 100 may be a heterogeneous network that includes BSs of different types, e.g., macro BS, pico BS, femto BS, relays, etc. These different types of BSs may have different transmit power levels, different coverage areas, and different impact on interference in the wireless network 100. For example, macro BS may have a high transmit power level (e.g., 20 Watts) whereas pico BS, femto BS, and relays may have a lower transmit power level (e.g., 1 Watt).

The wireless communication network 100 may support synchronous or asynchronous operation. For synchronous operation, the BSs may have similar frame timing, and transmissions from different BSs may be approximately aligned in time. For asynchronous operation, the BSs may have different frame timing, and transmissions from different BSs may not be aligned in time. The techniques described herein may be used for both synchronous and asynchronous operation.

A network controller 130 may couple to a set of BSs and provide coordination and control for these BSs. The network controller 130 may communicate with the BSs 110 via a backhaul. The BSs 110 may also communicate with one another, e.g., directly or indirectly via wireless or wireline backhaul.

The UEs 120 (e.g., 120 x, 120 y, etc.) may be dispersed throughout the wireless network 100, and each UE may be stationary or mobile. A UE may also be referred to as a mobile station, a terminal, an access terminal, a subscriber unit, a station, a Customer Premises Equipment (CPE), a cellular phone, a smart phone, a personal digital assistant (PDA), a wireless modem, a wireless communication device, a handheld device, a laptop computer, a cordless phone, a wireless local loop (WLL) station, a tablet, a camera, a gaming device, a netbook, a smartbook, an ultrabook, a medical device or medical equipment, a biometric sensor/device, a wearable device such as a smart watch, smart clothing, smart glasses, a smart wrist band, smart jewelry (e.g., a smart ring, a smart bracelet, etc.), an entertainment device (e.g., a music device, a video device, a satellite radio, etc.), a vehicular component or sensor, a smart meter/sensor, industrial manufacturing equipment, a global positioning system device, or any other suitable device that is configured to communicate via a wireless or wired medium. Some UEs may be considered evolved or machine-type communication (MTC) devices or evolved MTC (eMTC) devices. MTC and eMTC UEs include, for example, robots, drones, remote devices, sensors, meters, monitors, location tags, etc., that may communicate with a BS, another device (e.g., remote device), or some other entity. A wireless node may provide, for example, connectivity for or to a network (e.g., a wide area network such as Internet or a cellular network) via a wired or wireless communication link. Some UEs may be considered Internet-of-Things (IoT) devices.

In FIG. 1, a solid line with double arrows indicates desired transmissions between a UE and a serving BS, which is a BS designated to serve the UE on the downlink and/or uplink. A dashed line with double arrows indicates interfering transmissions between a UE and a BS.

Certain wireless networks (e.g., LTE) utilize orthogonal frequency division multiplexing (OFDM) on the downlink and single-carrier frequency division multiplexing (SC-FDM) on the uplink. OFDM and SC-FDM partition the system bandwidth into multiple (K) orthogonal subcarriers, which are also commonly referred to as tones, bins, etc. Each subcarrier may be modulated with data. In general, modulation symbols are sent in the frequency domain with OFDM and in the time domain with SC-FDM. The spacing between adjacent subcarriers may be fixed, and the total number of subcarriers (K) may be dependent on the system bandwidth. For example, the spacing of the subcarriers may be 15 kHz and the minimum resource allocation (called a “resource block” (RB)) may be 12 subcarriers (or 180 kHz). Consequently, the nominal FFT size may be equal to 128, 256, 512, 1024 or 2048 for system bandwidth of 1.25, 2.5, 5, 10 or 20 megahertz (MHz), respectively. The system bandwidth may also be partitioned into subbands. For example, a subband may cover 1.08 MHz (i.e., 6 resource blocks), and there may be 1, 2, 4, 8 or 16 subbands for system bandwidth of 1.25, 2.5, 5, 10 or 20 MHz, respectively.

While aspects of the examples described herein may be associated with LTE technologies, aspects of the present disclosure may be applicable with other wireless communications systems, such as NR.

NR may utilize OFDM with a cyclic prefix (CP) on the uplink and downlink and include support for half-duplex operation using time division duplexing (TDD). A single component carrier (CC) bandwidth of 100 MHz may be supported. NR resource blocks may span 12 subcarriers with a subcarrier bandwidth of 75 kHz over a 0.1 ms duration. Each radio frame may consist of 2 half frames, each half frame consisting of 5 subframes, with a length of 10 ms. Consequently, each subframe may have a length of 1 ms. Each subframe may indicate a link direction (i.e., DL or UL) for data transmission and the link direction for each subframe may be dynamically switched. Each subframe may include DL/UL data as well as DL/UL control data. UL and DL subframes for NR may be as described in more detail below with respect to FIGS. 6 and 7. Beamforming may be supported and beam direction may be dynamically configured. MIMO transmissions with precoding may also be supported. MIMO configurations in the DL may support up to 8 transmit antennas with multi-layer DL transmissions up to 8 streams and up to 2 streams per UE. Multi-layer transmissions with up to 2 streams per UE may be supported. Aggregation of multiple cells may be supported with up to 8 serving cells. Alternatively, NR may support a different air interface, other than an OFDM-based. NR networks may include entities such central units (CUs) and/or distributed units (DUs).

In some examples, access to the air interface may be scheduled, wherein a scheduling entity (e.g., a base station) allocates resources for communication among some or all devices and equipment within its service area or cell. Within the present disclosure, as discussed further below, the scheduling entity may be responsible for scheduling, assigning, reconfiguring, and releasing resources for one or more subordinate entities. That is, for scheduled communication, subordinate entities utilize resources allocated by the scheduling entity. Base stations are not the only entities that may function as a scheduling entity. That is, in some examples, a UE may function as a scheduling entity, scheduling resources for one or more subordinate entities (e.g., one or more other UEs). In this example, the UE is functioning as a scheduling entity, and other UEs utilize resources scheduled by the UE for wireless communication. A UE may function as a scheduling entity in a peer-to-peer (P2P) network, and/or in a mesh network. In a mesh network example, UEs may optionally communicate directly with one another in addition to communicating with the scheduling entity.

Thus, in a wireless communication network with a scheduled access to time-frequency resources and having a cellular configuration, a P2P configuration, and a mesh configuration, a scheduling entity and one or more subordinate entities may communicate utilizing the scheduled resources.

As noted above, a RAN may include a CU and DUs. A NR BS (e.g., gNB, 5G Node B, Node B, transmission reception point (TRP), access point (AP)) may correspond to one or multiple BSs. NR cells can be configured as access cells (ACells) or data only cells (DCells). For example, the RAN (e.g., a CU or DU) can configure the cells. DCells may be cells used for carrier aggregation or dual connectivity, but not used for initial access, cell selection/reselection, or handover. In some cases DCells may not transmit synchronization signals (SS), but in some cases DCells may transmit SS. NR BSs may transmit downlink signals to UEs indicating the cell type. Based on the cell type indication, the UE may communicate with the NR BS. For example, the UE may determine NR BSs to consider for cell selection, access, handover, and/or measurement based on the indicated cell type.

FIG. 2 illustrates an example logical architecture of a distributed radio access network (RAN) 200, which may be implemented in the wireless communication system illustrated in FIG. 1. A 5G access node 206 may include an access node controller (ANC) 202. The ANC may be a central unit (CU) of the distributed RAN 200. The backhaul interface to the next generation core network (NG-CN) 204 may terminate at the ANC. The backhaul interface to neighboring next generation access nodes (NG-ANs) may terminate at the ANC. The ANC may include one or more TRPs 208 (which may also be referred to as BSs, NR BSs, Node Bs, 5G NBs, APs, or some other term). As described above, a TRP may be used interchangeably with “cell.”

The TRPs 208 may be a DU. The TRPs may be connected to one ANC (ANC 202) or more than one ANC (not illustrated). For example, for RAN sharing, radio as a service (RaaS), and service specific ANC deployments, the TRP may be connected to more than one ANC. A TRP may include one or more antenna ports. The TRPs may be configured to individually (e.g., dynamic selection) or jointly (e.g., joint transmission) serve traffic to a UE.

The logical architecture 200 may be used to illustrate fronthaul definition. The logical architecture 200 may support fronthauling solutions across different deployment types. For example, the logical architecture 200 may be based on transmit network capabilities (e.g., bandwidth, latency, and/or jitter).

The logical architecture 200 may share features and/or components with LTE. The next generation AN (NG-AN) 210 may support dual connectivity with NR. The NG-AN 210 may share a common fronthaul for LTE and NR.

The logical architecture 200 may enable cooperation between and among TRPs 208. For example, cooperation may be preset within a TRP and/or across TRPs via the ANC 202. There may be no inter-TRP interface.

Logical architecture 200 may have a dynamic configuration of split logical functions. As will be described in more detail with reference to FIG. 5, the Radio Resource Control (RRC) layer, Packet Data Convergence Protocol (PDCP) layer, Radio Link Control (RLC) layer, Medium Access Control (MAC) layer, and a Physical (PHY) layers may be adaptably placed at the DU or CU (e.g., TRP or ANC, respectively).

FIG. 3 illustrates an example physical architecture 300 of a distributed RAN, according to aspects of the present disclosure. A centralized core network unit (C-CU) 302 may host core network functions. The C-CU 302 may be centrally deployed. C-CU functionality may be offloaded (e.g., to advanced wireless services (AWS)), in an effort to handle peak capacity.

A centralized RAN unit (C-RU) 304 may host one or more ANC functions. Optionally, the C-RU 304 may host core network functions locally. The C-RU 304 may have distributed deployment. The C-RU 304 may be close to the network edge.

A DU 306 may host one or more TRPs (edge node (EN), an edge unit (EU), a radio head (RH), a smart radio head (SRH), or the like). The DU may be located at edges of the network with radio frequency (RF) functionality.

FIG. 4 illustrates example components of the BS 110 and UE 120 illustrated in FIG. 1, which may be used to implement aspects of the present disclosure. The BS may include a TRP and may be referred to as a Master eNB (MeNB) (e.g., Master BS, primary BS). Master BS and the Secondary BS may be geographically co-located.

One or more components of the BS 110 and UE 120 may be used to practice aspects of the present disclosure. For example, antennas 452, Tx/Rx 454, processors 466, 458, 464, and/or controller/processor 480 of the UE 120 and/or antennas 434, processors 420, 430, 438, and/or controller/processor 440 of the BS 110 may be used to perform the operations 1000 described herein and illustrated with reference to FIG. 10 and complementary operations.

FIG. 4 shows a block diagram of a design of a BS 110 and a UE 120, which may be one of the BSs and one of the UEs in FIG. 1. For a restricted association scenario, the BS 110 may be the macro BS 110 c in FIG. 1, and the UE 120 may be the UE 120 y. The BS 110 may also be a BS of some other type. The BS 110 may be equipped with antennas 434 a through 434 t, and the UE 120 may be equipped with antennas 452 a through 452 r.

At the BS 110, a transmit processor 420 may receive data from a data source 412 and control information from a controller/processor 440. The control information may be for the Physical Broadcast Channel (PBCH), Physical Control Format Indicator Channel (PCFICH), Physical Hybrid ARQ Indicator Channel (PHICH), Physical Downlink Control Channel (PDCCH), etc. The data may be for the Physical Downlink Shared Channel (PDSCH), etc. The processor 420 may process (e.g., encode and symbol map) the data and control information to obtain data symbols and control symbols, respectively. The processor 420 may also generate reference symbols, e.g., for the PSS, SSS, and cell-specific reference signal (CRS). A transmit (TX) multiple-input multiple-output (MIMO) processor 430 may perform spatial processing (e.g., precoding) on the data symbols, the control symbols, and/or the reference symbols, if applicable, and may provide output symbol streams to the modulators (MODs) 432 a through 432 t. Each modulator 432 may process a respective output symbol stream (e.g., for OFDM, etc.) to obtain an output sample stream. Each modulator 432 may further process (e.g., convert to analog, amplify, filter, and upconvert) the output sample stream to obtain a downlink signal. Downlink signals from modulators 432 a through 432 t may be transmitted via the antennas 434 a through 434 t, respectively.

At the UE 120, the antennas 452 a through 452 r may receive the downlink signals from the base station 110 and may provide received signals to the demodulators (DEMODs) 454 a through 454 r, respectively. Each demodulator 454 may condition (e.g., filter, amplify, downconvert, and digitize) a respective received signal to obtain input samples. Each demodulator 454 may further process the input samples (e.g., for OFDM, etc.) to obtain received symbols. A MIMO detector 456 may obtain received symbols from all the demodulators 454 a through 454 r, perform MIMO detection on the received symbols if applicable, and provide detected symbols. A receive processor 458 may process (e.g., demodulate, deinterleave, and decode) the detected symbols, provide decoded data for the UE 120 to a data sink 460, and provide decoded control information to a controller/processor 480.

On the uplink, at the UE 120, a transmit processor 464 may receive and process data (e.g., for the Physical Uplink Shared Channel (PUSCH)) from a data source 462 and control information (e.g., for the Physical Uplink Control Channel (PUCCH) from the controller/processor 480. The transmit processor 464 may also generate reference symbols for a reference signal. The symbols from the transmit processor 464 may be precoded by a TX MIMO processor 466 if applicable, further processed by the demodulators 454 a through 454 r (e.g., for SC-FDM, etc.), and transmitted to the base station 110. At the BS 110, the uplink signals from the UE 120 may be received by the antennas 434, processed by the modulators 432, detected by a MIMO detector 436 if applicable, and further processed by a receive processor 438 to obtain decoded data and control information sent by the UE 120. The receive processor 438 may provide the decoded data to a data sink 439 and the decoded control information to the controller/processor 440.

The controllers/processors 440 and 480 may direct the operation at the base station 110 and the UE 120, respectively. The processor 440 and/or other processors and modules at the BS 110 may perform or direct, e.g., the execution of the functional blocks illustrated in FIG. 10, and/or other complementary processes for the techniques described herein. The processor 480 and/or other processors and modules at the UE 120 may perform or direct, e.g., the execution of the functional blocks illustrated in FIG. 10, and/or other complementary processes for the techniques described herein. The memories 442 and 482 may store data and program codes for the BS 110 and the UE 120, respectively. A scheduler 444 may schedule UEs for data transmission on the downlink and/or uplink.

FIG. 5 illustrates a diagram 500 showing examples for implementing a communications protocol stack, according to aspects of the present disclosure. The illustrated communications protocol stacks may be implemented by devices operating in a 5G system. Diagram 500 illustrates a communications protocol stack including a Radio Resource Control (RRC) layer 510, a Packet Data Convergence Protocol (PDCP) layer 515, a Radio Link Control (RLC) layer 520, a Medium Access Control (MAC) layer 525, and a Physical (PHY) layer 530. In various examples the layers of a protocol stack may be implemented as separate modules of software, portions of a processor or ASIC, portions of non-collocated devices connected by a communications link, or various combinations thereof. Collocated and non-collocated implementations may be used, for example, in a protocol stack for a network access device (e.g., ANs, CUs, and/or DUs) or a UE.

A first option 505-a shows a split implementation of a protocol stack, in which implementation of the protocol stack is split between a centralized network access device (e.g., an ANC 202 in FIG. 2) and distributed network access device (e.g., DU 208 in FIG. 2). In the first option 505-a, an RRC layer 510 and a PDCP layer 515 may be implemented by the central unit, and an RLC layer 520, a MAC layer 525, and a PHY layer 530 may be implemented by the DU. In various examples the CU and the DU may be collocated or non-collocated. The first option 505-a may be useful in a macro cell, micro cell, or pico cell deployment.

A second option 505-b shows a unified implementation of a protocol stack, in which the protocol stack is implemented in a single network access device (e.g., access node (AN), new radio base station (NR BS), a new radio Node-B (NR NB), a network node (NN), or the like.). In the second option, the RRC layer 510, the PDCP layer 515, the RLC layer 520, the MAC layer 525, and the PHY layer 530 may each be implemented by the AN. The second option 505-b may be useful in a femto cell deployment.

Regardless of whether a network access device implements part or all of a protocol stack, a UE may implement an entire protocol stack (e.g., the RRC layer 510, the PDCP layer 515, the RLC layer 520, the MAC layer 525, and the PHY layer 530).

FIG. 6 is a diagram showing an example of a DL-centric subframe 600. The DL-centric subframe 600 may include a control portion 602. The control portion 602 may exist in the initial or beginning portion of the DL-centric subframe 600. The control portion 602 may include various scheduling information and/or control information corresponding to various portions of the DL-centric subframe. In some configurations, the control portion 602 may be a physical DL control channel (PDCCH), as indicated in FIG. 6. The DL-centric subframe 600 may also include a DL data portion 604. The DL data portion 604 may be referred to as the payload of the DL-centric subframe 600. The DL data portion 604 may include the communication resources utilized to communicate DL data from the scheduling entity (e.g., UE or BS) to the subordinate entity (e.g., UE). In some configurations, the DL data portion 604 may be a physical DL shared channel (PDSCH).

The DL-centric subframe 600 may also include a common UL portion 606. The common UL portion 606 may sometimes be referred to as an UL burst, a common UL burst, and/or various other suitable terms. The common UL portion 606 may include feedback information corresponding to various other portions of the DL-centric subframe. For example, the common UL portion 606 may include feedback information corresponding to the control portion 602. Non-limiting examples of feedback information may include an ACK signal, a NACK signal, a HARQ indicator, and/or various other suitable types of information. The common UL portion 606 may include additional or alternative information, such as information pertaining to random access channel (RACH) procedures, scheduling requests (SRs), and various other suitable types of information. As illustrated in FIG. 6, the end of the DL data portion 604 may be separated in time from the beginning of the common UL portion 606. This time separation may sometimes be referred to as a gap, a guard period, a guard interval, and/or various other suitable terms. This separation provides time for the switch-over from DL communication (e.g., reception operation by the subordinate entity (e.g., UE)) to UL communication (e.g., transmission by the subordinate entity (e.g., UE)). One of ordinary skill in the art will understand that the foregoing is merely one example of a DL-centric subframe and alternative structures having similar features may exist without necessarily deviating from the aspects described herein.

FIG. 7 is a diagram showing an example of an UL-centric subframe 700. The UL-centric subframe 700 may include a control portion 702. The control portion 702 may exist in the initial or beginning portion of the UL-centric subframe. The control portion 702 in FIG. 7 may be similar to the control portion described above with reference to FIG. 6. The UL-centric subframe 700 may also include an UL data portion 704. The UL data portion 704 may sometimes be referred to as the payload of the UL-centric subframe 700. The UL portion may refer to the communication resources utilized to communicate UL data from the subordinate entity (e.g., UE) to the scheduling entity (e.g., UE or BS). In some configurations, the control portion 702 may be a physical UL control channel (PUCCH).

As illustrated in FIG. 7, the end of the control portion 702 may be separated in time from the beginning of the UL data portion 704. This time separation may sometimes be referred to as a gap, guard period, guard interval, and/or various other suitable terms. This separation provides time for the switch-over from DL communication (e.g., reception operation by the scheduling entity) to UL communication (e.g., transmission by the scheduling entity). The UL-centric subframe 700 may also include a common UL portion 706. The common UL portion 706 in FIG. 7 may be similar to the common UL portion 706 described above with reference to FIG. 7. The common UL portion 706 may additionally or alternatively include information pertaining to channel quality indicator (CQI), sounding reference signals (SRSs), and various other suitable types of information. One of ordinary skill in the art will understand that the foregoing is merely one example of an UL-centric subframe and alternative structures having similar features may exist without necessarily deviating from the aspects described herein.

In some circumstances, two or more subordinate entities (e.g., UEs) may communicate with each other using sidelink signals. Real-world applications of such sidelink communications may include public safety, proximity services, UE-to-network relaying, vehicle-to-vehicle (V2V) communications, Internet of Everything (IoE) communications, IoT communications, mission-critical mesh, and/or various other suitable applications. Generally, a sidelink signal may refer to a signal communicated from one subordinate entity (e.g., UE1) to another subordinate entity (e.g., UE2) without relaying that communication through the scheduling entity (e.g., UE or BS), even though the scheduling entity may be utilized for scheduling and/or control purposes. In some examples, the sidelink signals may be communicated using a licensed spectrum (unlike wireless local area networks, which typically use an unlicensed spectrum).

A UE may operate in various radio resource configurations, including a configuration associated with transmitting pilots using a dedicated set of resources (e.g., a radio resource control (RRC) dedicated state, etc.) or a configuration associated with transmitting pilots using a common set of resources (e.g., an RRC common state, etc.). When operating in the RRC dedicated state, the UE may select a dedicated set of resources for transmitting a pilot signal to a network. When operating in the RRC common state, the UE may select a common set of resources for transmitting a pilot signal to the network. In either case, a pilot signal transmitted by the UE may be received by one or more network access devices, such as an AN, or a DU, or portions thereof. Each receiving network access device may be configured to receive and measure pilot signals transmitted on the common set of resources, and also receive and measure pilot signals transmitted on dedicated sets of resources allocated to the UEs for which the network access device is a member of a monitoring set of network access devices for the UE. One or more of the receiving network access devices, or a CU to which receiving network access device(s) transmit the measurements of the pilot signals, may use the measurements to identify serving cells for the UEs, or to initiate a change of serving cell for one or more of the UEs.

Example Aperiodic Tracking Reference Signal

As discussed a UE 120 may receive signals transmitted by BS 110 on a DL. In order to determine parameters of a communication channel (e.g., on the DL) between UE 120 and BS 110, BS 110 may transmit one or more reference signals (RSs) to UE 120. The RS may be data that is known by both UE 120 and BS 110. Therefore, the UE 120 may compare the decoded received RS transmitted from BS 110, with the known RS to determine parameters of the communication channel in a process known as channel estimation. The channel estimation may be used to decode other data transmitted by the BS 110 to the UE 120 on the communication channel. For example, BS 110 may transmit a tracking reference signal (TRS) to UE 120. The UE 120 may utilize the TRS to perform one or more of time/frequency tracking, estimation of Doppler spread, estimation of delay spread, estimation of power delay profile, etc. of the channel between BS 110 and UE 120. In certain aspects, TRS is a device specific (e.g., UE-specific and transmitted on resources, such as resource blocks, allocated for a specific UE on the DL) RS and configured with higher-layer signaling (e.g., as part of a RRC signaling, media access control-control element (MAC-CE), downlink control information (DCI), etc.) in a device specific manner.

In certain aspects, the BS 110 is configured to transmit TRS periodically. For example, the BS 110 may transmit the TRS every Y number of slots (e.g., where Y is a positive integer). The BS 110 may further transmit the TRS over X number of slots (e.g., where X is a positive integer) for each transmission. Accordingly, the BS 110 may be configured to start transmission of TRS every Y number of slots and transmit TRS in X number of slots consecutively for each transmission, where X<Y.

Transmitting TRS periodically may not allow the UE 120 to account for sporadic changes in channel conditions (e.g., TX/RX conditions). For example, communication in NR may be bursty in nature, meaning that traffic (e.g., control and data transmissions) may be non-continuous and aperiodic in time and frequency. Further, adaptive link adaptation techniques may be used for the DL between BS 110 and UE 120 in order to maintain good link conditions between BS 110 and UE 120. For example, one or more of TX power control, precoding/antenna/beam switching, dynamic cell selection, etc., may be used as adaptive link adaptation techniques for the DL. Accordingly, due to changes in channel conditions (e.g., due to adaptive link adaptation techniques or other changes in channel conditions) channel estimation of channel-related parameters such as average delay, delay spread, Doppler spread, Doppler shift, etc. of the channel between BS 110 and UE 120 may change. Though UE 120 may be able to perform channel estimation to determine updated channel-related parameters based on periodically transmitted TRS and/or beam management procedures (e.g., for mmW based systems), the channel estimation may be inaccurate for signals received between the periods where TRS is transmitted by BS 110 and where channel conditions have changed. Further, beam management procedures may not have enough resources allocated for accurate channel estimation. This may lead to performance degradation as UE 120 may use inaccurate channel estimates to decode received signals from BS 110.

For example, FIG. 8 illustrates an example timeline of channel conditions of a DL between UE 120 and BS 110. Periodically, BS 110 transmits TRS at time 805, 810, and 815 as shown to UE 120 for UE 120 to perform channel estimation utilizing TRS. As shown, the channel conditions between BS 110 and UE 120 may change from a first set of channel conditions to a second set of channel conditions at time 820. Time 820 is between transmission of TRS at 805 and 815. Accordingly, during time period 825, UE 120 may have inaccurate channel estimates, and only be able to update channel estimates after time 815.

In order to improve performance of decoding signals at UE 120, techniques herein relate to BS 110 aperiodically transmitting TRS to UE 120, and UE 120 using the aperiodically transmitted TRS for channel estimation. In certain aspects, an aperiodic TRS may be a single burst TRS transmission transmitted aperiodically. The aperiodic TRS may be transmitted over the same number of slots as periodic TRS for each transmission, or may be transmitted over a different number of slots. The aperiodic TRS may be configured using the same configuration as the periodic TRS (e.g., the same higher layer signaling) or using a different configuration.

In certain aspects, BS 110 determines to transmit aperiodic TRS when channel conditions change on the DL. For example, FIG. 9 illustrates an example timeline of channel conditions of a DL between UE 120 and BS 110. Like shown in FIG. 8, periodically, BS 110 transmits periodic TRS at time 905, 910, and 915 as shown to UE 120 for UE 120 to perform channel estimation utilizing TRS. As shown, the channel conditions between BS 110 and UE 120 may change from a first set of channel conditions to a second set of channel conditions at time 920. Time 920 is between transmission of TRS at 905 and 915. Accordingly, BS 110 may determine to and transmit an aperiodic TRS at time 930, which is after the channel conditions change at time 920, but before the next periodic TRS transmission at time 915. UE 120 may utilize the aperiodic TRS to update channel estimates to accurately decode signals from BS 110.

In certain aspects, aperiodic TRS may not be transmitted every time channel conditions change, as this may introduce too much overhead for aperiodic TRS transmission. Further, channel conditions changes may not always be significant enough that channel estimation needs to be updated in order to accurately decode signals at the UE 120 from BS 110. Accordingly, in certain aspects, aperiodic TRS is transmitted only when one or more trigger conditions or thresholds are met.

In certain aspects, the transmission of aperiodic TRS may be triggered by the BS 110 or the UE 120. In certain aspects, BS 110 can indicate the presence or transmission timing of an aperiodic TRS to UE 120 in order for UE 120 to be able to determine when aperiodic TRS is transmitted, so it can receive the aperiodic TRS and perform channel estimation. In certain aspects, BS 110 may indicate the transmission timing of an aperiodic TRS along with the signaling (e.g., as part of RRC signaling, MAC-CE, DCI, etc.) for a channel condition change (e.g., one or more of TX power control, precoding/antenna/beam switching, dynamic cell selection, or other adaptive link adaptation techniques). In certain aspects, the BS 110 indicates the transmission timing of an aperiodic TRS separately from the signaling for channel condition changes.

In the case where the channel condition change is due to a beam switching event (e.g., in mmW systems), the triggering of an aperiodic TRS may also trigger transmission of a phase tracking reference signal (PTRS) by BS 110 to UE 120, used for phase tracking in the DL by UE 120. In certain aspects, PTRS has higher density in the time domain, but lower density in the frequency domain than TRS. Accordingly, by combining PTRS and TRS in a mmW system, the overhead for transmitting PTRS and TRS (e.g., time multiplexed together) can be reduced by balancing the load between PTRS and TRS.

In certain aspects, the presence of PTRS in transmissions from BS 110 to UE 120 is implicitly determined by UE 120 based on modulation coding scheme (MCS), bandwidth, and subcarrier spacing (SCS) of transmissions from BS 110. However, in certain aspects, the transmission of PTRS is explicitly triggered along with aperiodic TRS.

As discussed, in certain aspects, BS 110 may initiate a channel condition change on the DL (e.g., a beam switch from a first beam A to a second beam B for communication on the DL with UE 120). For example, at time N, BS 110 may send a beam switch command on the DL control channel to UE 120 indicating a new beam (beam B) to use for communication on the DL. The UE 120 may decode the beam switch command and acknowledge receipt of the beam switch command to BS 110. Accordingly, UE 120 may utilize beam B at time N+K₁, where K₁ is the time it takes for UE 120 to switch from beam A to beam B.

BS 110 may also switch to utilizing beam B at time N+K₁ so that UE 120 and BS 110 communicate on beam B. In certain aspects, BS 110 determines whether to transmit an aperiodic TRS at time N+K₁. For example, BS 110 determines if time N+K₁ is more than a threshold amount of time greater than the last beam management instance (e.g., channel estimation procedure) for beam B. If N+K₁ is more than a threshold amount of time greater than the last beam management instance, BS 110 determines to trigger aperiodic TRS. If N+K₁ is less than a threshold amount of time greater than the last beam management instance, BS 110 determines not to trigger aperiodic TRS. For example, BS 110 normally selects one of the beams from an active beam set at BS 110 to switch to. Such active beams in the active beam set are regularly (e.g., periodically) managed by a beam management process.

In certain aspects, regardless of the time separation between N+K₁ and the last beam management instance, BS 110 determines whether to transmit an aperiodic TRS at time N+K₁ based on whether the beam switch from beam A to beam B results in a large change in beam width and/or angle used for the DL between BS 110 and UE 120. For example, if the beam width, pattern and/or direction (e.g., angle of departure and/or arrival) changes by a threshold amount(s), BS 110 determines to trigger aperiodic TRS. If the beam width and/or angle does not change by a threshold amount(s), BS 110 determines not to trigger aperiodic TRS. In certain aspects, BS 110 may indicate trigger of aperiodic TRS at time N+K₁ (or another suitable time) together with or separately from a beam switching command sent to UE 120. BS 110 may indicate triggering of aperiodic TRS to UE 120 as discussed.

In certain aspects, though the beam switching is initiated by BS 110, UE 120 may request triggering of aperiodic TRS (e.g., based on the same criteria as discussed for BS 110). For example, the BS 110 may signal to UE 120 to initiate beam switching on the PDSCH. When UE 120 sends an ACK for the PDSCH transmission, it may also include a request for aperiodic TRS transmission (e.g., in PUCCH, MAC-CE, and/or scheduling request (SR)). BS 110 may then transmit the aperiodic TRS based on the request.

In certain aspects, UE 120 may initiate a channel condition change on the DL (e.g., a beam switch from a first beam A to a second beam B for communication on the DL with BS 110). For example, at time N, UE 120 may send a beam switch request to BS 110 (e.g., in PUCCH, MAC-CE, and/or SR) indicating a new beam (beam B) to use for communication on the DL. In certain aspects, UE 120 may also determine whether to include (e.g., based on the same criteria as discussed for BS 110) and then either include or not include a request for aperiodic TRS transmission along with (or separate from) the beam switch request. In certain aspects, if BS 110 accepts the beam switch request of UE 120, UE 120 and BS 110 may utilize beam B at time N+K₂, where K₂ is the time it takes for BS 110 to switch from beam A to beam B. Further, if the beam switching request includes (or the BS 110 separately receives from UE 120) a request for aperiodic TRS transmission, the BS 110 transmits aperiodic TRS at time N+K₂ (or another suitable time).

Though certain aspects are described as between a BS 110 and UE 120, certain aspects may also be used for communication between a BS 110 and another BS, or between UEs.

FIG. 10 illustrates example operations that may be performed by a wireless device (e.g., BS 110 or UE 120) for using an aperiodic TRS in accordance with aspects of the present disclosure.

Operations 1000 begin, at 1002, by determining a channel condition change of a downlink channel between a first wireless device and a second wireless device. Operations 1000 continue at 1004 by determining whether the channel condition change satisfies a trigger condition. Operations 1000 continue at 1006 by triggering transmission of an aperiodic tracking reference signal when the channel condition change satisfies the trigger condition.

FIG. 11 illustrates a communications device 1100 that may include various components (e.g., corresponding to means-plus-function components) configured to perform operations for the techniques disclosed herein, such as the operations illustrated in FIG. 10. The communications device 1100 includes a processing system 1114 coupled to a transceiver 1112. The transceiver 1112 is configured to transmit and receive signals for the communications device 1100 via an antenna 1120, such as the various signal described herein. The processing system 1114 may be configured to perform processing functions for the communications device 1100, including processing signals received and/or to be transmitted by the communications device 1100.

The processing system 1114 includes a processor 1108 coupled to a computer-readable medium/memory 1111 via a bus 1124. In certain aspects, the computer-readable medium/memory 1111 is configured to store instructions that when executed by processor 1108, cause the processor 1108 to perform the operations illustrated in FIG. 10, or other operations for performing the various techniques discussed herein.

In certain aspects, the processing system 1114 further includes a first determining component 1102 for performing the operations illustrated at 1002 in FIG. 10. Additionally, the processing system 1114 includes a second determining component 1104 for performing the operations illustrated at 1004 in FIG. 10. The processing system 1114 also includes a triggering component 1106 for performing the operations illustrated at 1006 in FIG. 10. The first determining component 1102, second determining component 1104, and triggering component 1106 may be coupled to the processor 1108 via bus 1124. In certain aspects, the first determining component 1102, second determining component 1104, and triggering component 1106 may be hardware circuits. In certain aspects, the first determining component 1102, second determining component 1104, and triggering component 1106 may be software components that are executed and run on processor 1108.

The methods disclosed herein comprise one or more steps or actions for achieving the described method. The method steps and/or actions may be interchanged with one another without departing from the scope of the claims. In other words, unless a specific order of steps or actions is specified, the order and/or use of specific steps and/or actions may be modified without departing from the scope of the claims.

As used herein, a phrase referring to “at least one of” a list of items refers to any combination of those items, including single members. As an example, “at least one of: a, b, or c” is intended to cover a, b, c, a-b, a-c, b-c, and a-b-c, as well as any combination with multiples of the same element (e.g., a-a, a-a-a, a-a-b, a-a-c, a-b-b, a-c-c, b-b, b-b-b, b-b-c, c-c, and c-c-c or any other ordering of a, b, and c).

As used herein, the term “determining” encompasses a wide variety of actions. For example, “determining” may include calculating, computing, processing, deriving, investigating, looking up (e.g., looking up in a table, a database or another data structure), ascertaining and the like. Also, “determining” may include receiving (e.g., receiving information), accessing (e.g., accessing data in a memory) and the like. Also, “determining” may include resolving, selecting, choosing, establishing and the like.

The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language of the claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” Unless specifically stated otherwise, the term “some” refers to one or more. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. § 112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.”

The various operations of methods described above may be performed by any suitable means capable of performing the corresponding functions. The means may include various hardware and/or software component(s) and/or module(s), including, but not limited to a circuit, an application specific integrated circuit (ASIC), or processor. Generally, where there are operations illustrated in figures, those operations may have corresponding counterpart means-plus-function components with similar numbering.

The various illustrative logical blocks, modules and circuits described in connection with the present disclosure may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device (PLD), discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any commercially available processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

If implemented in hardware, an example hardware configuration may comprise a processing system in a wireless node. The processing system may be implemented with a bus architecture. The bus may include any number of interconnecting buses and bridges depending on the specific application of the processing system and the overall design constraints. The bus may link together various circuits including a processor, machine-readable media, and a bus interface. The bus interface may be used to connect a network adapter, among other things, to the processing system via the bus. The network adapter may be used to implement the signal processing functions of the PHY layer. In the case of a user terminal 120 (see FIG. 1), a user interface (e.g., keypad, display, mouse, joystick, etc.) may also be connected to the bus. The bus may also link various other circuits such as timing sources, peripherals, voltage regulators, power management circuits, and the like, which are well known in the art, and therefore, will not be described any further. The processor may be implemented with one or more general-purpose and/or special-purpose processors. Examples include microprocessors, microcontrollers, DSP processors, and other circuitry that can execute software. Those skilled in the art will recognize how best to implement the described functionality for the processing system depending on the particular application and the overall design constraints imposed on the overall system.

If implemented in software, the functions may be stored or transmitted over as one or more instructions or code on a computer readable medium. Software shall be construed broadly to mean instructions, data, or any combination thereof, whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. Computer-readable media include both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. The processor may be responsible for managing the bus and general processing, including the execution of software modules stored on the machine-readable storage media. A computer-readable storage medium may be coupled to a processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. By way of example, the machine-readable media may include a transmission line, a carrier wave modulated by data, and/or a computer readable storage medium with instructions stored thereon separate from the wireless node, all of which may be accessed by the processor through the bus interface. Alternatively, or in addition, the machine-readable media, or any portion thereof, may be integrated into the processor, such as the case may be with cache and/or general register files. Examples of machine-readable storage media may include, by way of example, RAM (Random Access Memory), flash memory, ROM (Read Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable Programmable Read-Only Memory), EEPROM (Electrically Erasable Programmable Read-Only Memory), registers, magnetic disks, optical disks, hard drives, or any other suitable storage medium, or any combination thereof. The machine-readable media may be embodied in a computer-program product.

A software module may comprise a single instruction, or many instructions, and may be distributed over several different code segments, among different programs, and across multiple storage media. The computer-readable media may comprise a number of software modules. The software modules include instructions that, when executed by an apparatus such as a processor, cause the processing system to perform various functions. The software modules may include a transmission module and a receiving module. Each software module may reside in a single storage device or be distributed across multiple storage devices. By way of example, a software module may be loaded into RAM from a hard drive when a triggering event occurs. During execution of the software module, the processor may load some of the instructions into cache to increase access speed. One or more cache lines may then be loaded into a general register file for execution by the processor. When referring to the functionality of a software module below, it will be understood that such functionality is implemented by the processor when executing instructions from that software module.

Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared (IR), radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Blu-ray® disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Thus, in some aspects computer-readable media may comprise non-transitory computer-readable media (e.g., tangible media). In addition, for other aspects computer-readable media may comprise transitory computer-readable media (e.g., a signal). Combinations of the above should also be included within the scope of computer-readable media.

Thus, certain aspects may comprise a computer program product for performing the operations presented herein. For example, such a computer program product may comprise a computer-readable medium having instructions stored (and/or encoded) thereon, the instructions being executable by one or more processors to perform the operations described herein. For example, instructions for perform the operations described herein and illustrated in FIG. 10.

Further, it should be appreciated that modules and/or other appropriate means for performing the methods and techniques described herein can be downloaded and/or otherwise obtained by a user terminal and/or base station as applicable. For example, such a device can be coupled to a server to facilitate the transfer of means for performing the methods described herein. Alternatively, various methods described herein can be provided via storage means (e.g., RAM, ROM, a physical storage medium such as a compact disc (CD) or floppy disk, etc.), such that a user terminal and/or base station can obtain the various methods upon coupling or providing the storage means to the device. Moreover, any other suitable technique for providing the methods and techniques described herein to a device can be utilized.

It is to be understood that the claims are not limited to the precise configuration and components illustrated above. Various modifications, changes and variations may be made in the arrangement, operation and details of the methods and apparatus described above without departing from the scope of the claims. 

What is claimed is:
 1. An apparatus for wireless communications at a user equipment (UE), comprising: a processor; memory coupled with the processor; and instructions stored in the memory and executable by the processor to cause the apparatus to: receive periodically, from a network entity, a periodic reference signal; and while receiving periodically, from the network entity, the periodic reference signal, receive an aperiodic reference signal from the network entity.
 2. The apparatus of claim 1, wherein the instructions are executable by the processor to further cause the apparatus to receive an indication of triggering of the aperiodic reference signal from the network entity.
 3. The apparatus of claim 2, wherein the indication comprises a downlink control information (DCI).
 4. The apparatus of claim 1, wherein each of the periodic reference signal and the aperiodic reference signal is used for time or frequency tracking.
 5. The apparatus of claim 1, wherein the periodic reference signal and the aperiodic reference signal are received over a same number of resources.
 6. An apparatus for wireless communications at a network entity, comprising: a processor; memory coupled with the processor; and instructions stored in the memory and executable by the processor to cause the apparatus to: transmit periodically, to a user equipment (UE), a periodic reference signal; and while transmitting periodically, to the UE, the periodic reference signal, transmit an aperiodic reference signal to the UE.
 7. The apparatus of claim 6, wherein the instructions are executable by the processor to further cause the apparatus to transmit an indication of triggering of the aperiodic reference signal to the UE.
 8. The apparatus of claim 7, wherein the indication comprises a downlink control information (DCI).
 9. The apparatus of claim 6, wherein each of the periodic reference signal and the aperiodic reference signal is used for time or frequency tracking.
 10. The apparatus of claim 6, wherein the periodic reference signal and the aperiodic reference signal are transmitted over a same number of resources.
 11. A method for wireless communications at a user equipment (UE), comprising: receiving periodically, from a network entity, a periodic reference signal; and while receiving periodically, from the network entity, the periodic reference signal, receiving an aperiodic reference signal from the network entity.
 12. The method of claim 11, further comprising receiving an indication of triggering of the aperiodic reference signal from the network entity.
 13. The method of claim 12, wherein the indication comprises a downlink control information (DCI).
 14. The method of claim 11, wherein each of the periodic reference signal and the aperiodic reference signal is used for time or frequency tracking.
 15. The method of claim 11, wherein the periodic reference signal and the aperiodic reference signal are received over a same number of resources.
 16. A method for wireless communications at a network entity, comprising: transmitting periodically, to a user equipment (UE), a periodic reference signal; and while transmitting periodically, to the UE, the periodic reference signal, transmitting an aperiodic reference signal to the UE.
 17. The method of claim 16, further comprising transmitting an indication of triggering of the aperiodic reference signal to the UE.
 18. The method of claim 17, wherein the indication comprises a downlink control information (DCI).
 19. The method of claim 16, wherein each of the periodic reference signal and the aperiodic reference signal is used for time or frequency tracking.
 20. The method of claim 16, wherein the periodic reference signal and the aperiodic reference signal are transmitted over a same number of resources. 